The use of plasma technology for the synthesis of nanopowders and materials processing has attracted considerable attention over the past decade. The principal advantage of plasma technology resides in its ability to decouple the chemistry of the process from the energy level (i.e. temperature) at which a given chemical or physical transformation is carried out. In contrast to combustion flame reactors in which the reaction medium contains the combustion products, plasma technology constitutes a high temperature process in which it is possible to independently control the chemistry of the reaction process and the reaction temperature. Plasma reactors can be operated using an inert, an oxidizing or a reducing atmosphere at temperatures reaching 10,000 degrees Kelvin or higher.
A standard technique for the synthesis of nanopowders using plasma technology involves evaporating the nanopowder precursor, whether in the form of a solid or a liquid, followed by quenching the generated vapor under well controlled conditions (FIG. 1). In the quenching step, the vapours are cooled either through contact with a cold surface or through direct mixing with a cold gas (i.e. quench gas). In either case, the quenching process goes through a nucleation step followed by particle growth and agglomeration. The final particle size distribution of the nanopowder is directly dependent on the temperature field in the quench zone of the plasma reactor.
Due to the difficulty in keeping the vapours from contacting colder surfaces within the plasma reactor, particle condensation within the reactor is generally inevitable and represents an adverse problem often responsible for reactor blockage and loss of productivity. Moreover, such adverse particle condensation represents a potential for product loss in addition to becoming a potential source of contamination of the nanopowder product.
In a number of plasma applications, the quench gas can also be reactive in nature (i.e. be a reactant) thus giving rise to chemical andor physical modifications of the nanopowder product. A reactive quench gas has been widely used in the synthesis of metal oxide nanopowders as well as in the synthesis of nitride and carbide nanopowder materials.
A common challenge to plasma related processes, whether including reactive or passive quenching, whether for the purpose of producing nanopowder materials or merely for melting and consolidating materials, resides in the difficulty of controlling the temperature field within the reactor and thus the thermo-chemical conditions to which the materials are exposed.
The present specification refers to a number of documents, the content of which is herein incorporated by reference in their entirety.